Bimodal modulation

ABSTRACT

An apparatus and associated method employing a bridge circuit having first and second microactuators attached to the apparatus. An excitation source is configured to independently drive each of the microactuators. A computation module is connected to the bridge circuit and is configured to independently measure an electrical output of each microactuator. The computation module uses the electrical outputs to characterize a bimodal modulation of the apparatus.

SUMMARY

Various embodiments of the present technology are generally directed tothe construction and use of microactuator circuitry to detect motion ofan object.

Some embodiments of this disclosure contemplate an apparatus thatincludes a bridge circuit having first and second microactuatorsattached to the apparatus. An excitation source is configured toindependently drive each of the microactuators. A computation module isconnected to the bridge circuit and is configured to independentlymeasure an electrical output of each microactuator. The computationmodule uses the electrical outputs to characterize a bimodal modulationof the apparatus.

Some embodiments of this disclosure contemplate a method including stepsof: independently exciting first and second self-sensing microactuatorsthat are each attached to an apparatus; independently measuring anelectrical output of each microactuator; and using the electricaloutputs to characterize a bimodal modulation of the apparatus.

Some embodiments of this disclosure contemplate an apparatus thatincludes first and second microactuators attached to the apparatus.Circuitry is configured to independently drive each of themicroactuators, and is configured to independently measure an electricaloutput from each of the microactuators to characterize bimodalmodulation of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a top view of an actuator in a disc drive data storagedevice.

FIG. 2 diagrammatically depicts a top view of the microactuator in theneutral position in the disc drive in FIG. 1.

FIG. 3 is similar to FIG. 2 but depicting the microactuator havingrotated the head gimbal assembly (HGA).

FIG. 4 diagrammatically depicts the piezoelectric effect in the PZTelements in the microactuator in FIG. 3.

FIG. 5 is a side view of FIG. 2 in the neutral position of themicroactuator.

FIG. 6 is similar to FIG. 5 but depicting vertical-dominant modedeflection of the microactuator.

FIG. 7 diagrammatically depicts the piezoelectric effect in the PZTelements in the microactuator in FIG. 6.

FIG. 8 is a flowchart depicting steps in a method for performing in situtesting in accordance with embodiments of this technology.

FIG. 9 is a schematic depiction of circuitry employed to obtain thebimodal characterization of the HGA modulation in accordance withillustrative embodiments of this technology.

FIG. 10 is a bimodal frequency response function (FRF) signature of thedisc drive in FIG. 1.

FIG. 11 is an enlarged portion of the FRF signature in FIG. 10.

FIG. 12 is an enlarged portion of the FRF signature in FIG. 10.

FIG. 13 graphically depicts the PZT response at a constant resonantfrequency in relation to varying heater power to identify head/disccontact.

DETAILED DESCRIPTION

A wide variety of machines employ microactuators for precise control ofmoving parts. A “microactuator” for purposes of this descriptionincludes a device that mechanically deforms when subjected to anexternal excitation, such as a driving voltage, and that develops asense voltage in proportion to deformation. Because of this dualfunctionality these devices are commonly referred to as a self-sensingactuator. For purposes of this illustrative description themicroactuator can include a piezoelectric transducer (PZT), although thecontemplated embodiments are not so limited. In alternative embodimentsthe microactuaor can otherwise be constructed to include things such asa magnetorestrictive element or a piezomagnetic element and the like.

For example, FIG. 1 depicts part of a disc drive data storage devicethat employs a microactuator for movement control. The disc drive has arotatable actuator 100 that is precisely moved to position a read/writehead at its distal end in alignment with a data storage track formed inthe surface of a rotating magnetic recording medium 102. The read/writehead, which can be a perpendicular magnetic head or a lateral magnetichead, reads and writes information by detecting and modifying themagnetic polarization of the recording layer on the surface of thestorage disc 102. The actuator 100 has a central body 106 that isjournalled for rotation around an axis of rotation 108 of a bearing 110.A voice coil 112 extends from the body 106 where it is immersed in amagnetic field from opposing magnets (only the bottom magnet 114 isdepicted in FIG. 1 for clarity sake). An arm 116 extends from the body106 opposite the voice coil 112. A plate 118 is attached to the distalend of the arm 116, such as by the crimp 119 depicted in FIG. 1.

Dual stage motion control is provided in that the voice coil 112provides coarse position control and a microactuator 120 provides fineposition control of the read/write head. The microactuator 120 includesa pair of PZT elements 122, 124 that selectively move a head gimbalassembly (HGA) 128 relative to the body 106. The HGA 128 includes theplate 118, a bulkhead 126 against which the PZTs 122, 124 engage, a loadbeam 129, and a gimbal 130 extending from the load beam 129 that, inturn, supports the read/write head.

The read/write head has a slider that is aerodynamically designed to besupported upon an air bearing created by rotation of the storage disc102. The surface of the slider closest to the storage disc 102 isreferred to as an air bearing surface (ABS). The HGA 128 is designed toproperly maintain the ABS at a desired fly height and orientation apartfrom the surface of the storage disc 102. A heater 127 is provided inthe read/write head to control the fly height by pole tip protrusion.The HGA 128 also includes a flexible circuit (trace) 131 that transmitselectrical signals and power to the read/write head and otherelectronics such as the heater 127.

FIG. 2 diagrammatically depicts the PZT elements 122, 124 at a neutralposition corresponding to a non-driven excitation state. Controlledapplication of a driving voltage to the PZT elements 122, 124 deformsthem to rotate the HGA 128 relative to the body 106 as depicted in FIG.3. This arrangement can be constructed by poling the PZTs across thethickness (normally done in practice) and flipping the polaritiesbetween the two PZTs in FIG. 2. By the piezoelectric effect, applying adriving voltage that is opposite to the poled polarity of the PZT 124causes it to shorten, whereas applying the driving voltage that is thesame as the poled polarity of the PZT 122 causes it to lengthen.Shortening the PZT 124 and lengthening the PZT 122 causes thecounter-clockwise rotational displacement depicted in FIG. 3. Reversingthe polarity of the driving voltage imparts a clockwise rotationaldisplacement in similar fashion.

Driving the PZTs 122, 124 to achieve this rotational movement providesfine positioning control of the read/write head relative to the datastorage tracks; the primary purpose of the microactuator 120 in the discdrive. The skilled artisan readily understands that the fine positioningcan be achieved in alternative ways, regardless of the poled polaritiesof the PZTs 122, 124, such as by providing individual voltages to themand perhaps phase shifting one or both of the individual drivingvoltages.

The storage disc 102 operably rotates at high speeds, subjecting the HGA128 to forces and exciting resonance that can significantly alter theposition of the ABS. For example, these forces can distort the HGA 128enough to create a pitch static angle that alters the flying orientationof the read/write head. Modulation-related failures are typically causedby excitation of HGA resonances. These forces and resonances can makethe HGA 128 unstable for its intended purpose when it becomesunacceptably sensitive to disturbances (can be particle or contaminantinteractions, for example) at the head-disc interface. Suchsensitivities can cause modulation-related failures rendering the discdrive unreliable and short lived.

HGA resonance stems from a wide range of excitation mechanisms andfailure modes. The complex structure of the HGA results in a largenumber of structural modes, making it difficult to entirely design themout of the disc drive. Further, the modal response of the HGA can varysignificantly from part to part. For this reason modulation failures maybe experienced only on a portion of a drive population. This makesindividual characterization of the modal response of each individual HGAimportant. Therefore, what is needed is an in situ test that comparesthe HGA modulation to an expected threshold to identify characteristicHGA modulation issues during the manufacturing process and thereafter.It is to those improvements and others that the embodiments of thepresent technology are directed.

Referencing back to FIG. 3 momentarily, recall that the counterclockwiseHGA 128 rotation requires a shortening (compression) of the PZT 124 anda lengthening (tension force) on the PZT 122. External forces acting onthe HGA 128 in the offtrack dominant mode create the same result; theycompress the PZT 124 and lengthen the PZT 122. FIG. 4 diagrammaticallydepicts the PZTs 122, 124 with their polarities swapped in relation tothe longitudinal poling axis 129. The arrows 131 facing each otherrepresent the compressive force acting on the PZT 124 and the arrows 133facing away from each other represent the tensile force acting on thePZT 122. By the piezoelectric effect, the compressive force 131 actingupon the positively poled PZT 124 produces a positive sense voltage. Thetensile force 133 acting upon the negatively poled PZT 122 likewiseproduces a positive sense voltage. For purposes of this description andmeaning of the appended claims, when the two PZTs 122, 124 strain in thesame direction at the same time, as indicated by the polarity of theirsense voltages, then the mode is termed a symmetric mode.

FIG. 5 is a side view of FIG. 2 in the vertically-neutral position, andFIG. 6 depicts external forces acting on the HGA 128 in thevertically-dominant mode. In this case both PZTs 122, 124 are lengthenedas they are flexed downward. FIG. 7 is similar to FIG. 4 except that inthis case both of the PZTs 122, 124 are subjected to tensile forces,represented by the opposing arrows 135, 137 facing away from each otherdue to the flexing. By the piezoelectric effect, the tensile forceacting upon the positively poled PZT 124 produces a negative sensevoltage (−V). The tensile force acting upon the negatively poled PZT 122produces a positive sense voltage (+V). For purposes of this descriptionand meaning of the appended claims, when the two PZTs 122, 124 strain inopposite directions at the same time, as indicated by the polarity oftheir sense voltages, then the mode is termed an asymmetric mode.

FIG. 8 is a flowchart depicting steps in a method for performing an insitu HGA modulation test on a disc drive in accordance with illustrativeembodiments of the present technology. The method begins in block 150 byperforming in situ testing. For purposes of this description, themeaning of an “in situ” test is limited to testing that is performedusing only the hardware and logic in place within the device under test;in this example within the disc drive. The in situ test specificallydoes not include tests that include adding extraneous data collectiondevices to measure data or perform a function that the disc drive isincapable of performing with the components already in place for itsintended use as a data storage device. The in situ testing in block 150can be limited to only the HGA modulation testing of this technology orit can be included as part of other testing. For example, the in situtesting 150 can be performed as part of the drive certification stageduring manufacturing of the disc drive. Subsequently, the in situtesting 150 can be performed as part of a screening test performed ondisc drives that are returned from commercial use for repair orrefurbishing.

In block 152 the disc drive's bimodal (symmetric and asymmetric modes)frequency response function (FRF) signature is obtained. FIG. 9 is aschematic depiction of microactuator circuitry 200 for obtaining thebimodal FRF in accordance with illustrative embodiments of the presenttechnology. The microactuator circuitry 200 generally includes acomputation module 202 connected to a bridge circuit 204. In theseillustrative embodiments the bridge circuit 204 includes two separateWheatstone bridge circuits 206, 208 connected in parallel to a voltagesource 210. Each of the PZTs 122, 124 is schematically represented as avoltage component in series with a capacitance element. The PZT 122 isincluded in one branch of the Wheatstone bridge 206, and the PZT 124 isincluded in one branch of the Wheatstone bridge 208.

In these illustrative embodiments the PZTs 122, 124 are drivenindependently of each other by the excitation source, in this case thevoltage source 210. Driving the PZTs 122, 124 results in unbalancedvoltages V_(out1), V_(out2) that are each proportional to the strain inthe respective individual PZT 122, 124. The computation module 202 has asumming circuit 212 that independently measures the electrical output ofthe PZT 122 in terms of the differential voltage V_(a1)−V_(b1). For agiven excitation input V_(in), the voltage sensed in leg (C₂-C₃) is:

$V_{b\; 1} = {\frac{C_{3}}{C_{2} + C_{3}}V_{in}}$

Similarly, for the opposing leg (C₁-PZT) the sensed voltage is:

$V_{a\; 1} = {{\frac{C_{p}}{C_{1} + C_{p}}V_{in}} + V_{{pzt}\; 1}}$

Therefore, the unbalanced voltage in the Wheatstone bridge circuit 206is:

${V_{a\; 1} - V_{b\; 1}} = {{( {\frac{C_{p}}{C_{1} + C_{p}} - \frac{C_{3}}{C_{2} + C_{3}}} )V_{in}} + V_{{pzt}\; 1}}$

If C1=C2, C3≈Cp, then V_(out1)=V_(a1)−V_(b1)=V_(pzt1)

Similarly, the computation module 202 has another summing circuit 214that independently measures the electrical output of the PZT 124 interms of V_(out2)=V_(a2)−V_(b2)=V_(pzt2). The asymmetric modes arecalculated in terms of:V _(out-asymmetric) =V _(out1) +V _(out2)

The symmetric modes are calculated in terms of:V _(out-symmetric) =V _(out1) −V _(out2)

The PZTs 122, 124 are driven independently of each other by use of theseparate Wheatstone bridges. In these embodiments the computation module202 includes a voltage generator 211 capable of selectively alteringelectrical frequency of the driving voltage V_(in). The driving voltageis varied within a predetermined frequency range, and the resultant PZT122, 124 sense voltages are simultaneously sampled at each of thesefrequency points. The unbalanced voltages are recorded throughout thefrequency range to characterize the HGA 128 modulation by an FRFsignature. FIG. 10 graphically depicts an illustrative FRF signature 220for the frequency range between zero to 50,000 Hz.

Returning to FIG. 8, in block 154 the obtained FRF signature 220 can becompared to predefined thresholds to qualitatively characterize the discdrive's HGA modulation. For example, FIG. 11 depicts an enlarged portionof the FRF 220 in FIG. 10 at the 20 k Hz resonant frequency. The modalgain for the FRF 220 differs from that of a preexisting threshold FRF(or stored value, T) by a variance depicted by numeral 219. For anotherexample, FIG. 12 is similar to FIG. 11 except depicting the resonantfrequency for the FRF 220 differs from that of the threshold T by avariance depicted by numeral 221. The thresholds can be set by examiningthe modal response (FRF) of sample sets of HGAs and using statisticsrepresentative of this sample population. In these examples the modalgain values and/or resonant frequency values can be compared to expectedthreshold values, and if that comparison passes muster then the discdrive is deemed acceptable for commercial use. For example, the discdrive can be qualitatively approved if these values vary by no more thanX %:

${\frac{{{Gain}_{T} - {Gain}_{220}}}{{Gain}_{220}}*100} < X$

On the other hand, disc drives having statistically derived outliervalues can be identified and heuristic rules employed to sort andcorrespondingly further test or rework the disc drive under test.

Referring to FIG. 8, in block 156 the obtained FRF signature 220 can bestored in a computer memory for use as a historical baseline referenceof that disc drive's HGA modulation at the time of in situ testing. TheFRF signature 220 can be stored in memory either internal or external tothe disc drive.

In block 158 the computation module 202 tests the disc drive for properfly height to ensure no unexpected contact will occur between theread/write head and the storage media (head/disc contact). Preferably,the computation module 202 first ascertains a resonant frequency fromthe FRF signature 220, such as 222 at 20 k Hz depicted in FIG. 10. Thecomputation module 202 then drives the PZTs 122, 124 substantially atthat constant resonant frequency and varies the power to the heater 127to deflect the read/write head toward the storage media. FIG. 13graphically depicts the PZT 122, 124 response increases and thendecreases around the power setting P_(c) at which head/disc contact ismade. Empirical testing and statistical regression can be employed todefine a range 224 within which contact is expected at the selectedresonant frequency. The range 224 is defined between a lower limit power(P_(LL)) and an upper limit power (P_(UL)). If P_(c) is within thepredetermined range then the disc drive is deemed acceptable forcommercial use. In addition this information can be used to filter theservo inputs driving micro-actuation to avoid these resonances duringnormal drive operation.

It is to be understood that even though numerous characteristics ofvarious embodiments of the present disclosure have been set forth in theforegoing description, together with details of the structure andfunction of various embodiments, this detailed description isillustrative only, and changes may be made in detail, especially inmatters of structure and arrangements of parts within the principles ofthe present disclosure to the full extent indicated by the broad generalmeaning of the terms in which the appended claims are expressed.

What is claimed is:
 1. An apparatus comprising: a bridge circuitincluding first and second microactuators attached to the apparatus; anexcitation source configured to independently drive each of themicroactuators; and a computation module connected to the bridge circuitand configured to independently measure an electrical output of eachmicroactuator and to use the electrical outputs to characterize abimodal modulation of the apparatus.
 2. The apparatus of claim 1 whereinthe bridge circuit comprises two Wheatstone bridge circuits, eachWheatstone bridge circuit including one of the microactuators.
 3. Theapparatus of claim 2 wherein the excitation source comprises a voltagesource electrically connected to the bridge circuit.
 4. The apparatus ofclaim 3 comprising a voltage generator configured to vary frequency ofexcitation to characterize the bimodal modulation as a frequencyresponse function (FRF).
 5. The apparatus of claim 4 wherein thecomputation module is configured to compare one or more modal gains andcorresponding resonance frequencies in the FRF to correspondingpredetermined thresholds to qualitatively characterize the apparatus. 6.The apparatus of claim 3 wherein the Wheatstone bridge circuits areelectrically connected in parallel to the voltage source.
 7. Theapparatus of claim 6 wherein the microactuators are attached to a headgimbal assembly (HGA) supporting a read/write head adjacent a datastorage media, and wherein the computation module is configured tosimultaneously characterize the HGA modulation in an offtrack-dominantmode and in a vertical-dominant mode.
 8. The apparatus of claim 7wherein the computation module is configured to identify dominantresonant frequencies in the FRF.
 9. The apparatus of claim 8 wherein thevoltage generator is configured to constantly drive the microactuator atthe resonant frequency while varying the power to a heater on the HGA todetermine a heater power value at which the read/write head contacts thedata storage media.
 10. The apparatus of claim 9 wherein the computationmodule is configured to compare the heater power value to apredetermined threshold to qualitatively characterize the apparatus. 11.The apparatus of claim 9 wherein each of the microactuators isconfigured to produce a piezoelectric effect, wherein the microactuatorsstrain in the same direction at the same time in the vertically-dominantmode, and wherein the microactuators strain in opposite directions atthe same time in the offtrack-dominant mode.
 12. The apparatus of claim7 wherein the computation module is configured to compute thevertically-dominant modes by computing a difference between theelectrical outputs and to compute the offtrack-dominant modes bycomputing a sum of the electrical outputs.
 13. A method comprising:independently exciting first and second self-sensing microactuators thatare each attached to an apparatus; independently measuring an electricaloutput of each microactuator; and using the electrical outputs tocharacterize a bimodal modulation of the apparatus.
 14. The method ofclaim 13 wherein the exciting includes variably exciting theself-sensing microactuators and the characterization is a frequencyresponse function (FRF).
 15. The method of claim 14 wherein the excitingincludes exciting the microactuators at a predetermined dominantresonance of the apparatus, and includes measuring a clearance betweenthe apparatus and another object at the dominant resonant frequency. 16.The method of claim 15 wherein the clearance is indicated by varyingpower to a heater at the dominant resonant frequency.
 17. The method ofclaim 14 wherein the FRF is compared to a predetermined threshold toqualitatively characterize the apparatus.
 18. The method of claim 14wherein the microactuators are attached to a head gimbal assembly (HGA)supporting a read/write head adjacent a data storage media in a discdrive, and storing the FRF in a memory.
 19. The method of claim 18 wherethe FRF is a first FRF, subsequently characterizing the apparatus by asecond FRF, and comparing the second FRF to the first FRF toqualitatively characterize the apparatus.
 20. An apparatus comprising:first and second microactuators attached to the apparatus; circuitryconfigured to independently drive each of the microactuators, andconfigured to independently measure an electrical output from each ofthe microactuators to characterize bimodal modulation of the apparatus.